Evidence indicates that Parkinson's Disease (PD) patients are particularly impaired in the performance of complex movements that require coordination of several joints. Studying these impairments is of considerable practical importance, because movements performed in everyday life (reaching, grasping, pointing, lifting, etc) are essentially multi-joint and the lack of coordination is one of the most debilitating aspects of the disease. Many studies have hypothesized that impairments in multi-joint movements arise from difficulties patients have in controlling several joints simultaneously. Data obtained in our lab suggest that the reason for multi-joint movement distortions in PD is different. We argue that PD patients have difficulties in the regulation of interactive torques. These torques may be considered as the effect of constraints imposed by peripheral biomechanics on control of the limbs. Healthy older adults overcome these constraints at normal movement speeds. Our data suggest that PD patients are unable to deal with the biomechanical constraints during movements of moderate and even low speeds. The purpose of the present proposal is (1) to demonstrate that the inability to properly regulate interactive torques is the major cause of multi-joint movement disruptions in PD, (2) to establish how interactive torques affect control at individual joints in multi-joint movements of the patients, and (3) to examine plausible reasons for the inability of patients to coordinate muscle torques necessary to modulate interactive torques at the joints. Our experiments utilize various arm movements, which manipulate joint coordination patterns stressing different roles of interactive torques in the control of joint movements. The data obtained in our experiments will provide important scientific and clinical contributions into knowledge of basal ganglia dysfunctions.